I wouldn't count Heath Ledger's Joker, I would say "apart from Heath Ledger", he will win, and not by a small margin. I made this poll and Ledger had Hannibal Lecter, Keyser Soze, Hans Landa as competitors (stronger entries than the other Joker) and look at the result:https://www.imdb.com/poll/wgCbJe3P-Vk/results

Most poll takers would likely vote the same. The Best 'Joker'
poll suggestion is at least dissimilar enough that it asks about an
individual actor's performance instead of the entire movie. One can like
the movie without liking the performance or visa-versa.

May I suggest one small edit? (just adding the article "a" like below)

Whose performance as The Joker in a live action movies or TV series do you think is the best?

I just saw 'Joker' and while I was not very impressed with the film, Phoenix's performance was not only the best portrayal of Joker that I've seen (I've seen Nicholson's and Ledger's) but one of the most exceptional performances that I've ever seen. Wow.

I wouldn't even include Joaquin Phoenix in this poll. His character is Arthur Fleck, not Jack Napier. Jack Napier is a cunning, anarchistic, highly functioning psychopath; Arthur Fleck is just a neurotic psychopath